The spinal cord is the main pathway for information connecting the
brain and peripheral nervous system.

The human spinal cord is protected by the bony spinal column shown to the
left. The spinal column is made up of bones called vertebrae. Although the spinal column is somewhat
flexible, some of the vertebrae in the lower parts of the spinal column
become fused. Hear it... Vertebrae

Example of a vertebra

The spinal cord is located in the vertebral foramen and is made up of 31
segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.
A pair of spinal nerves leaves each segment of
the spinal cord.

Magnetic Resonance Image (MRI) of the Spinal
Cord

The length of the spinal cord is about 45 cm in men and 43 cm in women.
The spinal cord is shorter than the length of the bony spinal column;
the spinal cord extends down only to the last of the thoracic vertebrae.
Nerves that extend from the spinal cord from the lumbar and sacral levels
must run in the vertebral canal for a distance before they leave the
vertebral column. This collection of nerves in the vertebral canal is
called the cauda equina (which means "horse
tail").

Receptors in the skin send information to the spinal cord through the
spinal nerves. The cell bodies for these nerve fibers are located in the
dorsal root ganglion. The nerve fibers enter the spinal cord through the
dorsal root. Some fibers make synapses with other neurons in the dorsal
horn, while others continue up to the brain. Many cell bodies in the
ventral horn of the spinal cord send axons through the ventral root to
muscles to control movement.

In the figures below, note the differences in the shape and size of the
spinal cord at different levels. The dark gray color in each segment
represents "gray matter." If you use your imagination, you can see that
the gray matter looks similar to an H or
a butterfly. Nerve cell bodies are located in the gray matter.
Surrounding the gray matter is white matter (lighter color shading) - this
is where the axons of the spinal cord are located.

Spinal Cord
Segments - Outlines

Spinal Cord Segments -
Photographs (not to scale)

Compare the relative amount of gray and white matter at each level of
the spinal cord. In the cervical segment, there is a relatively large
amount of white matter. This pattern is caused by the many axons going up
to the brain from all levels of the spinal cord AND there are many axons
traveling from the brain down to different segments of the spinal cord.
In lower segments of the spinal cord, there is less white matter because
there are fewer axons traveling to and from the brain.

There are also differences in the gray matter. In the cervical segment,
the ventral horn (the lower half of the segment) is enlarged. Also in the
lumbar segment that is illustrated, the ventral horn is large. These
segments are those that contain motor neurons that control movement of the
arms (cervical segment) and legs (lumbar segment).

Try it!

Can you guess which spinal cord goes
with which animal? Try it with this:

The first cervical
vertebra is also called the atlas. Atlas was one
of the Titans in Greek mythology. After a fight with Perseus, Atlas was
turned to stone and had to carry the weight of the Earth and heavens on
his shoulders. Therefore, the first cervical vertebra was named the atlas
because it carries the weight of the head.

The human spinal column is made up of 33 bones: 7 vertebrae in the
cervical region, 12 in the thoracic region, 5 in the lumbar region, 5 in
the sacral region and 4 in the coccygeal region. However, in adults the
bones in the sacral region join to form one bone called the sacrum; the
bones in the cocygeal region join to form one bone called the coccyx.

Did you know?

Giraffes and
humans have SEVEN vertebrae in their necks.

Giraffe Vertebra

Most mammals have only seven neck bones (cervical vertebrae),
but there are exceptions to this rule. The manatee and the two-toed sloth have only
SIX cervical vertebrae, the ant bear has EIGHT cervical vertebrae and the
three-toed sloth has NINE cervical vertebrae.